Add local SSH permission troubleshooting and resolution steps

[RainyCityCoder]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

docs/content/authentication/troubleshooting-ssh/error-permission-denied-publickey.md

What part(s) of the article would you like to see updated?

When troubleshooting my own connectivity issues I found that local SSH file permissions were the cause of non-connectivity, and steps to remediate this weren't available in this documentation.

Additional information

No response

[RainyCityCoder]

I'm planning to submit a PR for this shortly.